There is an increasing interest in portable electronic devices capable of operating over the full voltage range of their battery to optimize operating time of the device. Since integrated circuits included in those devices need regulated voltages, the usage of DC-DC converters able to work over this wide voltage range becomes more and more crucial. Buck-boost converters are DC-DC converters which may fulfill the latter requirements. They are able to provide a regulated-down voltage by operating in buck mode when the battery voltage is higher than the required output voltage, e.g. when the battery is fully charged. The other way round, buck-boost converters are able to provide a regulated up-voltage by operating in boost mode when the required output voltage is higher than the battery voltage, e.g. when the battery is discharged.
FIG. 6 shows a popular architecture of a buck-boost converter which is based on four switches connected to a power coil. As illustrated in FIG. 6, in buck mode, switches S1 and S2 are activated with a given duty cycle while switch S4 is permanently on and switch S3 is permanently off. The current flow through the buck-boost converter is illustrated by two dashed arrows 1, 2 which indicate the two different phases occurring during each clock cycle. In the boost mode illustrated in FIG. 7, only switches S3 and S4 are activated while switch S1 is permanently on and switch S2 is permanently off. When the battery has an intermediate voltage, the buck-boost converter can work in buck-boost mode, by activation of all switches in three consecutive phases during a single clock cycle. In FIG. 8, these three phases are indicated by three dashed arrows 1, 2, 3.
FIG. 9 depicts a possible buck-boost control implementation for regulating the output voltage of the buck-boost converter. The control implementation comprises a feedback loop for determining a duty cycle for switching switches S1 and S2 (denoted as duty-buck) and a duty cycle for switching switches S3 and S4 (denoted as duty-boost) based on the output voltage of the converter. The respective duty cycles of the different phases are determined by the comparison of two internal voltages: the error voltage, which results from the amplification of the difference between the output voltage and the target voltage, and a voltage ramp generated by a ramp generator synchronized and periodically reseted by a master clock.
A problem when designing buck-boost converters is to provide a mechanism for determining in which mode (buck mode, boost mode, or buck-boost mode) the converter shall operate to provide the requested output voltage under the requested current load. This mechanism should also exhibit low power losses and smooth transitions between the different modes.
A simple approach is illustrated in FIG. 10 and comprises monitoring the input voltage using two comparators to determine in which operation mode the DC-DC converter should operate. If the input voltage VIN is higher than the target voltage VOUT, the buck mode is selected and the DC-DC converter is forced to operate in this mode. If the input voltage VIN is lower than the target voltage VOUT, the boost mode is selected. The drawback of this approach is that the mode selection does not take into account the load current and the voltage drop across the switches. Since the status of the regulation loop (such as e.g. the present duty cycles) is not taken into account, it is helpful to set a buck-mode voltage threshold (denoted as buck vth) and a separate boost-mode voltage threshold (denoted as boost vth) to account for a maximum output current. This can lead to the setting of a wide buck-boost mode operation area. This architecture, by construction, is not optimal for all current and voltage ranges and can create large output voltage glitches when a change of mode occurs.